Carbon anode protection in aluminum smelting cells

ABSTRACT

In the operation of a cell for the electrolytic reduction of Al2O3 dissolved in a cryolite bath of aluminum metal utilizing a carbon anode, the improvement wherein there is provided at the anode an atmosphere containing water in amounts effective for preventing anode dusting.

United States Patent. [191 .Sleppy et al.

[451 Dec. 17, 1974 CARBON ANODE PROTECTION IN ALUMINUM SMELTING CELLSInventors: William C. Sleppy, Belleville, 0].;

Ronald J. Campbell, Apollo, Pa.

[73] Assignee: Aluminum Company of America,

Pittsburgh, Pa.

[22] Filed: June 28,1973

[21] Appl. No.: 374,803

[52] US. Cl. 204/67 [51] Int. Cl. C22d 3/12 [58] Field of Search 204/67[56] References Cited UNITED STATES PATENTS 2,947,673 8/1960 Sem et all204/67 2,464,267 3/1949 Short 204/67 3,509,030 4/ l 970 Gooding 204/673,696,008 l()/l972 Lcvitan 204/67 Primary Examiner.lohn H. MackAssistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Daniel A.Sullivan, Jr.

[5 7] ABSTRACT In the operation of a cell for the electrolytic reductionof A1 0 dissolved in a cryolite bath of aluminum metal utilizing acarbon anode, the improvement wherein there is provided at the anode anatmosphere containing water in amounts effective for preventing anodedusting.

4 Claims, 2 Drawing Figures PATENTEI] DEE] U974 SHEET 2 0F 2 Hili 'lARGON TANK CARBON ANODE PROTECTION IN ALUMINUM SMELTING CELLS BACKGROUNDOF THE INVENTION The present invention relates to the operation ofaluminum reduction cells utilizing carbon anodes.

Depending upon operating conditions, consumption of carbon anodes inHall-Heroult process cells ranges from one-third to three-quarters of apound of carbon per pound of aluminum produced. The preferred conditionsare those leading to the stoichiometric minimum consumption, 0.33 lbs.C/lb. Al, predicted by the net cell reaction:

In experiments on closing off the space above the electrolyte bath of aHall-Heroult cell from the air, there has arisen an accumulation ofcarbon scum on the bath surface and a distribution of carbon dustthroughout the electrolyte. This carbon scum and dust is caused by adeterioration of the carbon anodes. The phenomenon is referred to asanode dusting.

Carbon scum causes alumina feeding problems. The scum has made itimpossible to replenish alumina consumed during electrolysis. As thedissolved alumina content of the bath decreases, scum formationaccelerates. Carbon dust and scum increases the bath viscosity andhinders diffusion of oxygen-bearing ions to the anode, thus limitinganode current densities and affecting the heat balance of the cell.Increases in the viscosity and density of the bath lower the currentefficiency and contribute to poor metal coalescence. The carbon in thescum and dust is not available for reaction with oxygen at the anode andso the gross consumption of carbon is increased by dusting. Because ofcarbon scum,

the bath agitation supplied by anode bubble evolution is reduced and thetendency for electrolyte to solidify at the metal pad-bath interfaceincreases. Ultimately, enough carbon dust can be distributed throughoutthe bath in closed cells to cause electronic conduction and completeloss of metal production. These conditions must be avoided forsuccessful operation of an enclosed cell.

Prolonging the life of anodes will not only decrease carbon consumption,but in the case of pre-baked anodes will decrease the amount of anodebutts to be recycled to the production of additional anodes and therebydecreases problems attendant upon evolution of fluorides during bakingof anodes.

SUMMARY OF THE INVENTION In view of the above, it is an object of thepresent invention to provide a method for preventing the phenomenon ofanode dusting, particularly in closed cells.

This, as well as other objects which will become apparent in thediscussion that follows, are achieved, according to the presentinvention, by a process for producing aluminum wherein a carbon anode ofa cell for the electrolytic reduction of Al O dissolved in a cryolitebath to aluminum metal is provided with an atmosphere containing waterin amounts effective for preventing anode dusting.

GENERAL ASPECTS OF THE INVENTION I It is believed that anode dusting iscaused by atmolite Attack by atmolite on exposed carbon anode surfacesis particularly a problem when using closed cells, e.g. cells which areclosed at the top by a plate.

The atmosphere containing water can be provided by surrounding anodesurfaces located above the electrolytic bath with a skirt spaced fromsuch surfaces, and injecting water vapor into the space between theskirt and the anode surfaces. Alternatively the water vapor can beprovided by closing off a space above the electrolyte bath anddissolving in the electrolyte bath alumina containing enough water thatsufficient water vapor is evolved and rises into the closed space toafford the desired protection. Such alumina can be produced by heatingalumina trihydrate to the desired water content by well-knowntechniques; eg using rotary kilns. It is desirable to feed the aluminainto the bath close to the anode or anodes.

It is believed that the amount of water vapor to be provided in anatmosphere around an anode surface depends to some extent on the amountof atmolite to be neutralized. In general, it is desirable to provide atleast enough water vapor to react the atmolite stoichiometrically withthe water vapor in accordance with the equation NaAlF, 3/2 H O NaF /z A10 3HF BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational,cross-sectional, broken-away view of a Soderberg anode type cell for usein the present invention.

FIG. 2 is a schematic representation, in cross-section, of bench-scaleequipment for illustrating a part of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably in the practice ofthe present invention, A1 0 is electrolytically decomposed to aluminummetal in an electrolyte bath between an anode and a cathodic interfacefonned between aluminum metal and the electrolyte bath, the bathconsisting essentially of Al O NaF. and AIR; and having a weight ratioNaF to AlF up to 1.1:]. The bath is maintained at an operatingtemperature greater than 40C above the cryolite liquidus temperature ofthe bath and effective for preventing bath crusting in interfacial areasbetween the bath and aluminum metal. The cryolite liquidus temperatureis that temperature at which cryolite first begins to crystallize oncooling the bath.

While the bath may consist only of Al O NaF, AlF it is possible toprovide in the bath at least one halide compound of the alkali andalkaline earth metals other than sodium in an amount effective forreducing the liquidus temperature of the bath below that which it wouldhave if only Al O NaF, and AlF were present. Suitable alkali andalkaline earth metal halides are LiF, CaF and MgF In a preferredembodiment, the bath contains lithium fluoride in an amount between 1and A preferred practice for maintaining the alumina concentration aselectrolytic reduction proceeds is to add, substantially continuously,directly to the molten vbath, an alumina which has a total water of 8 to20 percent, more preferably 10 to 18 percent. The surface area maypreferably be in the range 135 to 180 m /g. The total water is a measureof the water in the alumina and is defined herein as follows:

Expose a sample of alumina to 100 percent humidity for several hours,then equilibrate the sample at 44 percent relative humidity, 25C, for 18hours, then accurately weigh the sample, then ignite it to l,lC, thenweigh again. The loss in sample weight on going from the equilibratedstate at 44 percent relative humidity to the ignited state after heatingat 1,100C, divided by the sample weight at l,l00C, and multiplied by 100is the percent total water.

Surface area is measured by the Brunauer-Emmett- Teller method. SeeStephen Brunauer, P. H. Emmett, Edward Teller, J. of Am. Chem. Soc, V.60, Pgs. 309-l9, 1938.

The use of alumina of the high water content of the present invention iscontrary to the commonly-held view set forth at p. 34 of The ChemicalBackground of the Aluminum Industry by Pearson, published by The RoyalInstitute of Chemistry in 1955, that alumina used in electrolyticproduction of aluminum should be moisture-free.

Further illustrative of the present invention are the followingexamples:

EXAMPLE I With reference to FIG. 2, there is shown a graphite crucible51 having a non-conducting, refractory lining 52 with a hole 53 at itslower end. A molten aluminum metal pad 54 sits in the bottom of thealumina crucible and contacts the graphite crucible 52 to be inelectrical contact with cathode lead 55. Resting on pad 54 iselectrolyte bath 56 containing-4 wt. 70 Al O and NaF AlF at a bathweight ratio NaF/AlF 0.8. The electrolyte bath is at 900C. Carbon(prebaked, petroleum coke) anode 57 is immersed in the electrolyte bathto provide an electrical current density of amperes per inch on theanode. Aluminum anode skirt 58 surrounds the anode 57 as shown and issealed at its top by plug 59 provided with orifices for the passage ofanode lead 60 and gas flow pipe 61. Appropriate piping is provided forallowing varied amounts of argon gas to flow from tank 62 throughimpinger bottle 63 containing water 64 surrounded by an ice water bath65. Thus, the argon gas fed into the space between the anode 57 and theskirt 58 contained water vapor picked up by -the argon from the water inthe bottle 63. In operation of the cell to produce aluminum, carbonconsumption was 0.33 to 0.38 pounds per pound of aluminum produced at acurrent efficiency of lOO percent in 29 to 4l ampere-hour tests usingwater vapor shielding for preventing anode dusting. With 4 to 22 torrwater partial pressure in the argon, no carbon froth or scum wasdetected. When the impinger bottle 63 was bypassed so that only argonmoved down around anode 57, a carbon scum formed on the surface of thebath 56, and electronic conduction through the carbon scum occurred.

EXAMPLES H AND III Aluminum was produced in the cell of FIG. 1. Themaximum dimensions of the steel shell 20 in the horizontal were 18' 6''X 10' 2". Its maximum height was 3 9". The maximum dimensions of themolten aluminum metal pad 21 in the horizontal were 17' 8" X 9' 4". Theelectrolyte bath had the same maximum dimensions as the metal pad.

A mica mat 22 was provided between the steel shell 20 and graphite block23 for the purpose of preventing current flow through shell 20. Matthicknesses of from 6 to 20 mils have been used.

The pad 21 of molten aluminum was supported on carbonaceous cathodeblock lining 24 and carbonaceous tamped lining 25. The carbonaceouslinings were supported on an alumina fill 26, there being interposedbetween the tamped lining and the till some quarry tile 27. A layer ofred brick 28 was provided between the graphite block 23 and quarry tile27'.

FIG. 1 is a representative vertical section through the cell and it willbe realized that, for instance, similar graphite blocks 23 would appearin other elevational sections through the cell.

The anode 29 was a Soderberg-type carbon anode. The composition chargedto form this self-baking .anode was 31 percent pitch of softening pointequals 98lO0C (cube-in-air method) and 69 percent petroleum coke. Thecoke fraction was 30 percent coarse, 16 percent intermediate, and 54percent fine, the size distributions of the coarse, intermediate, andfine coke being given in Table I.

Table l Coke Size Distribution Cumulative 7r Greater Than Sieve Size Thecathode current was supplied through steel collector bars, such as bar30, to the block lining 24. The current supply is indicated by the plusand minus signs on the anode and on collector bar 30 respectively.

The space above the bath 31 was sealed from the surrounding air by aclosure 32, including a cast iron manifold 33, Ceraform Refractory board34, which is a soft (for obtaining a good seal) fibrous electrical andheat insulating board available from the Johns-Manville Co., steel shell35, steel plate 36, and fire clay brick, eg 50% M 0 and 50% $0,, 37.Within shell 35 there was provided a castable 38 serving a primarilyinsulative function and a castable 39, e.g. calcium-aluminatebondedtabular alumina, selected for its refractory properties. The particularheat transfer situation was chosen to maintain the upper surface 45 ofbath 31 substantially in molten condition, i.e. free of any crusting.

Alumina is charged from hopper 40 through a fill valve and feederassembly 41 of the type disclosed in US. Pat. No. 3,681,229 issued Aug.1, 1971 to R. L. Lowe entitled Alumina Feeder. Measured quantities ofalumina are fed onto the exposed molten bath surface through Inconel-600pipe 42. The distance between the bottom of pipe 42 and the top of bath31 is about 1 foot. The feeder 41 is a shot-type feeder, i.e. separatequantities of alumina are fed at timed intervals. In Examples 11 and111, two feeders 41 were used, and these fed-in alumina approximatelyevery 5 minutes, the quantities of alumina being adjusted to maintainthe desired alumina concentration in the bath. It takes about 1 minuteto discharge the alumina increments which were about 1,500 grams. Pipe42 is directed so as to impinge alumina onto the bath 31 where gas 44 isrising alongside the anode. This assures that the water evolved from thecharged alumina protects the anode against production of carbon dusttherefrom. This practice also promotes dissolution because of the bathagitation caused by the gas evolution. By charging the alumina in linewith a spike row (spikes 45a, b, and 0 lie in a vertical plane parallelto the plane of FIG. 1, which plane also contains pipe 42) in theSoderberg anode (cracks usually occur in the anode in line with spikerows), the dissolution rate is enhanced by the additional gas evolutionoccurring at the cracks. Feeders 41 were operated using air as thefluidizing medium, it being recognized that this represents a smallleakage of air past cover 32 to the bath.

The particular alumina used for Examples 11 and 111 had a total water of16.95 percent. This alumina was 98 plus 325 mesh and its water contentalone was sufficient to prevent anode dusting, i.e. a decomposition ofthe anode such that carbon particles build up in and on the bath.

The production data for Examples 11 and 111 are presented in Tables 11to IV.

Table 11 Pot Production Data Example No. Data Name 11 111 N.M. notmeasured Table 111 Pot Electrical Data Example No. 11 111 Data NameVolts/Pot 5.13 5.17 Average Ampcres 66.874 72,207 Kilowutts/Pot 343. 1'373.3 Ohmic Voltage Drop in Bath 1.70 1.68

Table IV Pot-Bath Data Example No.

Data Name I 111 Wt.-% CaF- 3.11 3.17 Wt A1 0,; 4.09 4.00 Wt.-% AlF 48.9745.08 Wt.-% LiF 5.61 10.165 Wt.-% NaF 38.13 36.94 Wt.-% Mg? .38 .28Liquidus emperaturc. "C 882 906 Calculated Wt.-Ratio NaF/AlF .78 .82Calculated Wt.-% C olite 63.4 61.9 Calculated Excess Al 3 23.4 20.5 BathOperating Temperature, C 898 922 "Eutectic Temperature, C 799 814Conductivity. ohm"'inches' 4.87 5.67 Bath Depth, inches 8.26 7.62 MetalDepth, inches 6.02 6.25

With special reference to Table IV, the excess A1F indicates thequantity of AlF above that present under the heading cryolite, formula3NaF.AlF In each of Examples I1 and 11], A1 0 would be the firstsubstance to crystallize on going below the given liquidus temperature.The eutectic temperature provides an estimate of the cryolite liquidustemperature in this case. The eutectic temperature is determined byfinding the liquidus temperature for progressively decreasing A1 0content, correspondingly increasing NaF AlF and constant bath ratioNaF/AIF and'selecting the minimum liquidus temperature on the basis ofthe resulting group of liquidus temperature values. The A1 0 in solutionis that at the particular bath operating temperature. Conductivity datais likewise for the given operating temperature.

Gases evolved from the Soderberg anode (e.g. hydrocarbons), fluoridesfrom the bath, and anode reaction gas (e.g. CO were vented from cover 32through an opening (not shown) and passed through a burner to burn thehydrocarbons. Because it is difficult to provide an absolute sealing ofthe bath from the air using cover 32, i.e. leaks can be present in cover32, a pressure of 0.03 to 0.1 inch of H 0, measured negatively fromatmospheric pressure, is maintained between the cover 32 and the burner,in order to prevent fume leakage from the cover 32. The burned gaseswere then fed to a scrubber system.

EXAMPLE IV This Example illustrates how a suitable alumina product asused in Examples 11 and 111 can be produced.

Bayer-process alumina hydrate was treated in a kiln to produce kilnactivated alumina suitable for use in the process of the presentinvention as follows. Kiln dimensions were 360 feet length and 9 /2 feetinner-diameter. Residence time of the material in the kiln was 1 to 1V2hours. The charged hydrate moved countercurrent to the combustion gasesintroduced into the lower end of the kiln. A maximum temperature of 400to 500C was achieved 10 to 15 feet inside the lower end of the kiln.Natural gas was burned at a rate of 6,500 cubic feet (standardtemperature and pressure) per hour to produce the combustion gases. Thisnatural gas flow rate was selected by testing the product for thedesired total water. The volume ratio of air to gas was 10:1. An aluminahaving a 12.5 percent total water was produced. Anywhere from 88 toweight of the particles had a size greater than 325 mesh.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

it will likewise be recognized that the action of water in the presentinvention will be subject to energyrelated laws such as rates ofreaction and chemical equilibrium constants and that anode dusting needonly be prevented to an extent such that there be no dustingrelatedimpairment of cell operation.

All percentages given herein are in percent by weight unless indicatedotherwise.

What is claimed is:

1. In the operation of a cell for the electrolytic reduction of A1dissolved in a cryolite bath to aluminum metal utilizing a carbon anode,the improvement comprising providing at the anode an atmospherecontaining water in amounts effective for preventing anode dusting.

2. In the operation of a cell as claimed in claim 1, the improvementfurther being characterized by a closing of the top of the cell.

3. In the operation of a cell as claimed in claim 1, the improvementbeing further characterized by the providing of a skirt around theanode, said atmosphere being provided within said skirt.

4. In the operation of a cell as claimed in claim 1, the improvementbeing further characterized in that the amount of water is at leastenough for reacting atmolite stoichiometrically according to theequation NaAlF, 3/2 H O NaF k A1 0 3HF.

UNITED STATES PATENT OFFICE CERTEFECATE OF CGRRECTION Patent No.3,855,086 Dated December 17, 1974 Inventor-(5) C. et 8.1

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:'

In the Abstract, After bath" change of line 2 to --to--.

Signed and sealed this 11th day of February 1975.

(SEAL) Attest:

' C. MARSHALL DANN RUTH MASON Commissioner of Patents Attesting Officer7 and Trademarks FORM P0405, I Y I.1sco\vm4-mc wave-Pee ".5. GOVERNMENTPRINTING OFFICE 7 I9, 0-35-3,

1. IN THE OPERATION OF CELL FOR THE ELECTROLYTIC REDUCTION OF AL2O3DISSOLED IN A CRYOLITE BATH T ALUMINUM METAL UTILIZING A CARBON ANODE,THE IMPROVEMENT COMPRISING PROVIDING AT THE ANODE AN ATMOSPHERECONTAINING WATER IN AMOUNTS EFFECTIVE FOR PREVENTING ANODE DUSTING. 2.In the operation of a cell as claimed in claim 1, the improvementfurther being characterized by a closing of the top of the cell.
 3. Inthe operation of a cell as claimed in claim 1, the improvement beingfurther characterized by the providing of a skirt around the anode, saidatmosphere being provided within said skirt.
 4. In the operation of acell as claimed in claim 1, the improvement being further characterizedin that the amount of water is at least enough for reacting atmolitestoichiometrically according to the equation NaAlF4 + 3/2 H2O -> NaF +1/2 Al2O3 + 3HF.